Cans are typically either two-piece or three-piece cans. In the case of a two-piece can, a can body is formed by punching a metal plate to form a cylinder closed at one end. Once the can is filled, the open end is then typically closed by seaming a lid to the can body. In the case of a three-piece can, a can body, open at both ends, is formed by rolling and seaming a metal plate. A first end is closed by typically seaming a lid to the can body. Once the can is filled and the second end is closed by typically seaming a lid to the can body.
FR 2639561 discloses a metal can and a method of manufacturing the same, the metal can comprising an internal annular flange to which a peelable lid can be heat-sealed in order to hermetically seal an interior space of the can. The peelable lid has a heat sealable layer which is used to hermetically bond the lid to the flange. An alternative heat sealing approach could involve providing adhesive around the upper surface of the flange and/or around the under surface of the peelable lid, heating the flange and applying downward pressure.
For some markets, the type of metal can described in FR 2639561 may be perfectly adequate. However, for more specialised markets, for example the infant formula market (i.e. metal cans that are used to store baby milk powder), where the seals are required to have very high performances even in the most hostile of environments, these known cans may not be suitable.
Before being introduced into the market, such specialised metal cans are required to undergo a series of stringent tests to ensure that they meet the necessary safety requirements. For example, metal cans may be tested to ensure that they do not lose their hermetic seal even when stored at high ambient temperatures such as 45° C. for periods in excess of 3 months and with a pressure difference from inside to outside of the can, for example of 700 mbar (70 kPa). Typically the external pressure is standard (ambient) air pressure and the internal pressure is negative, often referred to as a “vacuum”. Under such conditions, it has been found that metal cans made in accordance with those described in the prior art are prone to suffering from “creep” in the seal. Creep is the tendency for the peelable lid to be pulled inward across the flange. This can reduce the effectiveness of the seal between the peelable lid and the flange, and in some cases may cause the seal to fail completely. Metal cans that suffer from creep in this way cannot be used in the infant formula market as the seal is not deemed to seal the product to a high enough standard.